Rather than using a physical, mechanical switch, a transistor can act as an electronic switch, using signals to turn it on or off. Karen goes over what transistors are, how they work, and some different types of transistors: Bipololar junction transistors (BJT), NPN transistors, PNP transistors, and darlington transistors. Connect with Karen on element14: http://bit.ly/2EASjxT

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Previously, we’ve talked about how diodes work. Silicon diodes have a pn junction. Bipolar junction transistors or BJTs are bipolar because they have two pn junctions. BJTs are essentially two diodes in a single package. The two main types are NPN and PNP transistors. NPN transistors have two ntype regions on either side of one ptype region, while PNP transistors have two ptype regions, on either side of one ntype region. Bipolar transistors have 3 leads, one going to each region. Typically, the middle layer is the base. Ptype in an NPN, and ntype in a PNP. One of the other layers form the emitter and the third, the collector. These are labelled B, E, and C. On the circuit symbol, the arrow is always on the emitter, so we can tell which lead is the emitter and which is the collector by seeing which one has the arrow. The NPN transistor symbol has an arrow on the emitter pointing out, while the PNP transistor symbol has an arrow on the emitter pointing in. Transistors act as an electronic switch, conducting current across the collectoremitter path when a voltage is applied to the base. The switch is off if there is no base voltage present. When base voltage is present, the switch is on. We know from our diodes lesson, that diodes require a forward voltage of 0.7V before they are turned "on" allowing current to flow. In a standard NPN transistor, when 0.7V is applied between the base and the emitter, the transistor “turns ON”, allowing current to flow from the collector to the emitter. With an NPN transistor, we normally bias the device so that the collector voltage is positive with respect to the emitter. The voltage across these two points is referred to as the CollectorEmitter Voltage or VCE. If you connect the base to be positive with respect to the emitter, the voltage is referred to as the BaseEmitter voltage, or VBE. For a PNP transistor, rather than needing a minimum of 0.7V on the base of the transistor, there needs to be a minimum difference of 0.7V between the VCE, collectoremitter voltage, and VBE, baseemitter voltage. If the circuit power supply is a 9V battery, the baseemitter voltage would need to be no more than 8.3V for the transistor to turn “on” and allow current to flow between the collector and emitter. If the baseemitter voltage is 8.6V, a difference of 0.4V, the transistor would be off and no current would flow. If the baseemitter voltage is 7V, the difference of 2V is greater than 0.7V so the transistor would be “on”, allowing current to flow between the emitter and collector.